Robots are multi-body systems whose dynamics is nonlinear and highly coupled. Robotic control is most frequently accomplished with a position control system. This position control is realized at the joint level, where each joint is treated independently and coupled dynamic effects between joints are ignored. A typical position controller at each joint is implemented using PID control with high gains. Since this position controller cannot account for the dynamics of the system, the dynamic coupling effects are treated as a disturbance. This limits the performance that can be achieved by the system in the case of high speed precise trajectory tracking and compliant motion.
One approach for addressing this problem is to provide a torque controlled robot. The input torques for this system can be designed to accomplish the robot desired task as well as to compensate for nonlinear dynamic coupling of the system. This provides the robot with higher performance in position tracking as well as in compliant motion. Although torque controlled robots are preferred to achieve high performance control, most robots have embedded position control.
Since providing direct torque control for robots tends to be difficult, methods to improve the performance of position controlled robots have been investigated. For example, U.S. Pat. No. 5,834,917 considers a robot having a disturbance detector which detects a disturbance torque, where the disturbance torque is used to correct a position input. U.S. Pat. No. 5,742,138 considers a method for “floating” a robot by compensating for static external forces (e.g., gravity). Improved control of a position controlled robot is also considered in US 2004/0128030.
It would be an advance in the art to provide improved control of position controlled robots to more closely approach the desirable performance provided by closed loop control of both position and torque. In particular, it would be an advance to provide dynamic (i.e., time varying) torque control of a position-controlled robot.